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Influence of Chemical Interactions on the Surface-Enhanced Infrared Absorption Spectrometry of Nitrophenols on Copper and Silver Films Gregory T. Merklin† and Peter R. Griffiths* Department of Chemistry, University of Idaho, Moscow, Idaho 83844-2343 Received August 19, 1996. In Final Form: July 29, 1997X The surface-enhanced infrared spectrometry of nitrophenols on copper and silver films has been investigated. When molecules with strong electron-withdrawing groups, such as nitro groups, are adsorbed to coinage-group metals, sizeable changes in the infrared spectrum are observed. These changes can be attributed to donor-acceptor interactions between the nitro group and the metal which change the structure of the molecule. These chemical changes cause spectroscopic changes which influence the suitability of surface-enhanced infrared absorption for use in hyphenated techniques.
Introduction Surface-enhanced infrared absorption spectrometry has been a subject of some interest in the years since Hartstein et al.1 first observed that p-nitrobenzoic acid adsorbed to very thin silver films exhibited a 20-30-fold enhancement of infrared absorption. Badilescu and co-workers2 have investigated the nature of the effect through the use of o-, m-, and p-nitrobenzoic acids in an effort to determine the nature of surface enhancement, with particular attention to the role of geometric isomerism. Osawa et al.3 have also investigated the role of geometric isomerism in an investigation of the surface-enhanced spectra of m- and p-nitrobenzoic acid. The cause of the surface enhancement has not yet been completely elucidated. Interaction of the oscillating dipole of the adsorbed molecule with surface plasmons of the silver film was initially proposed by Hartstein.1 Osawa and Ikeda3 proposed that part of the enhancement is due to the local electric field enhancement around the small particles comprising the metal island film, particularly at the edges of the particles where the radius of curvature is small. This theoretical description was developed further by Osawa et al.4 in a later study. They modeled the metal island film (which at these thicknesses is not continuous) as a set of metal particles which they assumed to be prolate spheroids of rotation. The electromagnetic properties of such a spheroid coated with a uniform layer of an absorbing dielectric were calculated, and the results of these calculations were used in an effective-medium model to determine the dielectric constants of the composite metal/organic film. Osawa and Ikeda3 also acknowledged the presence, in principle, of other contributions to the surface-enhancement effect. One possibility that they cited was the orientation of dipoles on the metal surface and the consequent enhancement of the absorption, relative to that of the randomly oriented molecule, due to alignment of the dipole moment change of certain modes with the perpendicular electric field at the surface of the metal * Author to whom correspondence should be addressed. † Current address: Department of Chemistry, Stanford University, Stanford, CA 94305-5080. X Abstract published in Advance ACS Abstracts, October 15, 1997. (1) Hartstein, A.; Kirtley, J. R.; Tsang, J. C. Phys. Rev. Lett. 1980, 45, 201-4. (2) Badilescu, S.; Ashrit, P. V.; Truong, V.-V.; Badilescu, I. I. Appl. Spectrosc. 1989, 43, 549-52. (3) Osawa, M.; Ikeda, M. J. Phys. Chem. 1991, 95, 9914-19. (4) Osawa, M.; Ataka, K.-I.; Yoshii, K.; Nishikawa, Y. Appl. Spectrosc. 1993, 47, 1497-1502.
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particle. They also referred to charge transfer effects as described by Devlin and Consani5 for Raman spectrometry as possibly being responsible. In this case the enhancement would be caused by vibronic coupling of vibrational modes with charge-transfer excitations between the molecule and the metal surface. Osawa and Ikeda also suggested that the “chemical effect” may be due to changes in vibrational polarizability of the molecule due to chemical interactions with the surface of the metal, as it has been observed that vibrational polarizability of CO is increased on chemisorption to metal surfaces.6,7 Spectroscopic differences between different chemical species have also been observed in surface-enhanced Raman spectroscopy (SERS) spectra of N2 compared to CO8 and in electron energy-loss spectra of CO adsorbed to metals.9 Badilescu et al.2 noted that the observed enhancement appeared to be strongest for those groups within the molecule that were strongly polarizable, e.g., the carboxylic acid and nitro groups of the nitrobenzoic acids as opposed to the C-H stretching and bending modes, but did not discuss why this might be. This observation supports the arguments made in favor of the enhanced vibrational polarizability model but still leaves the possibility of other types of chemical interactions. A possible enhancement mechanism that has not been explored is the alteration in charge distribution, and thus dipole moment, as a result of chemical changes caused by the chemisorption of a molecule on the metal surface. As part of a study in the mechanisms responsible for surfaceenhanced absorption, we have undertaken an investigation of the surface-enhanced infrared spectrometry of different isomers of nitrophenols and dinitrophenols. Experimental Section Thin films of copper or silver (Johnson Matthey, Ward Hill, MA) were deposited on infrared-transparent substrates. CaF2 (McCarthy Scientific, Fullerton, CA) and Ge (Lattice Materials, Bozeman, MT) were used as the substrates for transmission measurements. The Ge disks were polished with 6-µm diamond paste and 0.05-µm alumina (Buhler, Lake Bluff, IL) and rinsed with deionized water. The metal films were generated via (5) Devlin, J. P.; Consani, K. J. Phys. Chem. 1981, 85, 2597-8. (6) Persson, B. N. J.; Ryberg, R. Phys. Rev. B 1981, 24, 6954-6970. (7) Dumas, P.; Tobin, R. G.; Richards, P. Surf. Sci. 1986, 171, 555578. (8) Moskovits, M.; DiLella, D. P.. In Surface Enhanced Raman Scattering; Chang, R. K., Furtak, T. E., Eds.; Plenum: New York, 1982. (9) Schmeisser, D.; Demuth, J. E.; Avouris, P. Chem. Phys. Lett. 1982, 87, 324.
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physical vapor deposition in a vacuum chamber held at a pressure of 10-6 Torr. The thickness of the deposited layer was monitored with a quartz crystal oscillator (Kronos, Torrance, CA, Model ADS-200). The copper films were 5 nm thick and the silver films 10 nm thick. Typical deposition rates were 0.5-0.7 nm/min. Organic Films. 2-Nitrophenol, 3-nitrophenol, 4-nitrophenol, 2,4-dinitrophenol, 2,5-dinitrophenol, 2,6-dinitrophenol, and 2,4,6trinitrophenol (Aldrich, Milwaukee, WI) and 3,4-dinitrophenol (Fluka, Ronkonkoma, NY) were used as supplied; 0.001% solutions were made in HPLC-grade methanol (Fisher, Pittsburgh, PA). The organic films were generated by depositing an aliquot of the solution (typically 20 µL) onto the surface of the metal-coated substrate with a syringe and allowing the solvent to evaporate under cover. The films thus generated were calculated to be approximately 1 nm thick, based on the amount of material used and the area of the substrate (12 mm diameter disk). To obtain unenhanced spectra, films were also cast from 0.1% solutions onto uncoated germanium substrates. Spectra were obtained on either a Bomem (Quebec City, Canada) MB-100 or a Perkin-Elmer (Norwalk, CT) 1800 FTIR spectrometer equipped with a Spectra-Tech (Stamford CT) IRPlan infrared microscope. The spectra were obtained in normalincidence transmission at 4 cm-1 resolution, using a liquidnitrogen-cooled mercury-cadmium-telluride detector. Solution spectra of the compounds were obtained in a 0.5% solution in chloroform (Fisher) in a 0.05 mm transmission cell with potassium bromide windows.
Results and Discussion As the metal fims were exposed to air prior to the application of the organic solutions, it is possible that a layer of oxide may form. We believe that this is not necessarily an impediment to the chemisorption of the phenol, which could react via an acid-base reaction of the type CuO + (Ph-OH)2 f Cu(O-Ph)2 + H2O. 4-Nitrophenol. The spectra of 4-nitrophenol on a silver film and in solution are shown in Figure 1. The spectra are significantly different, with numerous bands changing position and intensity. The two bands at 1617 and 1598 appear to have collapsed into a single band, at 1580 cm-1. In place of two bands at 1520 and 1500 cm-1, one band appears at 1483 cm-1. The band at 1340 cm-1 shifts only slightly (∆ν ) 4 cm-1), and the band at 1277 cm-1 has increased greatly in intensity relative to the band at 1342 cm-1. The band at 1111 cm-1 appears little changed. (We have frequently observed the dispersive bandshapes seen in the surface-enhanced infrared absorption (SEIRA) spectrum and believe that their presence or absence may be related to the rate at which the metal film is deposited. However, we do not as yet have a conclusive explanation for them.) The spectra of 4-nitrophenol in solution and as a film cast on bare germanium are shown in Figure 2. While there are differences in the spectra, they are relatively small compared to the differences observed between the solution and SEIRA spectra. Nitro-Group Frequencies. Symmetric NO2 Stretch. The band at 1342 cm-1 is almost certainly that resulting from the symmetric NO2 stretch and is increased in intensity relative to what would be expected from a cast film on a bare substrate. The band at 1272 cm-1 in the solution spectrum can also be assigned to vibrations involving the symmetric NO2 stretching mode. Bellamy10 notes that subsidiary bands can appear near the main 1340-cm-1 band in a few cases but also notes that they are generally weak, except for p-nitroaniline, where the bands are approximately equal in intensity. Epstein et al.11 have examined the effects of electrondonating substituents in the infrared spectra of nitroaro(10) Bellamy, L. J. The Infra-red Spectra of Complex Molecules 2nd ed.; Chapman and Hall: London, 1980; Vol. 2. (11) Epstein, L. M.; Shubina, E. S.; Ashkinaze, L. D.; Kazitsyna, L. A. Spectrochim. Acta 1982, 38A, 317-22.
Figure 1. Spectra of 4-nitrophenol (A) on a 5-nm copper film, (B) on a 10-nm silver film, and (C) in 0.05% CHCl3 solution.
Figure 2. Spectra of 4-nitrophenol (A) on an uncoated germanium substrate and (B) in a 0.05% CHCl3 solution.
matic compounds. They noted that in solution, coordinating solvents tended to increase the intensity of this subsidiary band and move it to higher frequencies. The subsidiary band was observed as low as 1280 cm-1 for the un-ionized compound in benzene solution and at 1277 cm-1 for the nitrophenolate anion in dimethyl sulfoxide (DMSO). From this evidence, it can be concluded that the band at 1277 cm-1 is the low-frequency component of the symmetric NO2 stretch. Epstein found that as the electrondonating character of the substituent para to the nitro group increased, the low-frequency component of this doublet increased in intensity and moved to higher frequencies, and the 1340-cm-1 band decreased in intensity and moved to lower frequencies. For a very strongly electron-donating substituent (e.g., OPbPh3 ) in a coor-
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Figure 3. Resonance structures for the 4-nitrophenolate anion.
dinating solvent (e.g., DMSO), the high-frequency component of this pair of bands disappeared completely. They also noted that the overall absorption intensity of the symmetric NO2 stretching bands increased with increasing electron-donating character, either when observing the intensity of the low-frequency component or when using the sum of the two components. This argument implies that part of the absorption enhancement that is seen for p-nitrophenol on silver is due to donor-acceptor interactions that may be stabilizing the nitrophenolate anion. The spectroscopic consequences of this can be understood in terms of resonance structures of the p-nitrophenolate anion (see Figure 3). The consequences of the π-electron delocalization will be a reduced frequency for the NO2 stretch, because of the reduced order of the N-O bonds. In addition, the increased charge separation between the nitrogen and the oxygen atoms will result in an increased dipole moment change and, thus, increased absorption intensity. The irregular, asymmetric profile of the 1270-cm-1 band immediately suggests that it may be comprised of not one but several superimposed bands. From resonance Raman (RR) and surface-enhanced RR data, Ni and co-workers12 assigned the 1284-cm-1 band as coupled NO2 and Ph-N stretches, and assigned the Ph-O stretch to 1270 cm-1. These assignments are consistent with our observations of the surface-enhanced infrared spectra. Antisymmetric NO2 stretch. From the effects described above, one might expect the antisymmetric stretch to intensify and move to lower frequencies. Thus the band observed at 1483 cm-1 in the surface-enhanced spectrum could be assigned to this mode. This wavenumber is rather low for this mode, as the lowest frequency observed by Kross and Fassel13 for the antisymmetric stretch was 1501 cm-1 for p-nitrophenoxide. Bellamy10 noted that νas can be as low as 1487 cm-1, but did not mention which compound gave rise to that spectrum. The location of this band in the SEIRA spectrum could be rationalized as being a consequence of an extremely strong electrondonation effect. Were this the case, one would also expect the symmetric band to behave in a more extreme fashion, perhaps along the lines of the strongest electron donors in Epstein’s study. Assignment of the 1487-cm-1 band as the antisymmetric NO2 stretching mode raises other problems. It is generally acknowledged that SEIRA spectra exhibit a surface selection rule,4 in that vibrations whose dipole moment change are parallel to the surface of the particles comprising the thin metal film will not be able to interact with the electric field of the incident radiation, which will be perpendicular to the surface of the metal and thus will (12) Ni, F.; Thomas, L.; Cotton, T. M. Anal. Chem. 1989, 61, 888-94. (13) Kross, R. D.; Fassel, V. A. J. Am. Chem. Soc. 1956, 78, 4225-9.
Figure 4. Normal modes of benzene: ν8a, ν8b, ν19a, and ν19b.
be forbidden. In a study of p-nitrothiophenol14 we have observed that this selection rule applies for the thiol. We believe that this is also true for p-nitrophenol. Ni and co-workers12 investigated the surface-enhanced resonance Raman scattering of a series of nitrophenols, and they concluded that the molecule is adsorbed perpendicular to the surface, which is consistent with our results. Furthermore, the absence of the high-frequency component of the 1617-1598 cm-1 doublet leads us to the same conclusion. This band is usually assigned as having B2 symmetry; if the molecule is adsorbed perpendicularly, the band will have its dipole moment change parallel to the surface of the metal, and thus be forbidden. On the other hand, if the molecule is adsorbing obliquely on the surface, then that band will reduce to the same symmetry species as the low-wavenumber component, and thus be able to share intensity with it.15 It will also have a dipole moment change with a component perpendicular to the metal surface and thus become surface-selection-rule allowed. We thus conclude that is unlikely that the 1487cm-1 band is the antisymmetric NO2 stretch. Phenyl Ring Frequencies. Four bands appear in the region of the infrared spectrum between 1600 and 1300cm-1 that can be attributed to vibrations of the benzene ring. These, according to Wilson’s16 nomenclature, are ν8a, ν8b, ν19a, and ν19b. The normal modes corresponding to these vibrations are shown in Figure 4. The band at 1580 cm-1 can be assigned to the ν8a ring frequency of the phenyl group. Its presence, and strong enhancement, can be attributed to donor-acceptor interactions between the silver-phenolate group at one end of the molecule and the nitro group at the other end. Katritzky and Topsom15 investigated the influence of donor-acceptor interactions as part of their study of factors affecting the intensity of infrared bands in conjugated systems. They found that differences in polarity of substituent groups would increase the intensity of this band and that substituents which disturbed the π system of a benzene ring so as to form a quinoid structure (see Figure 3) increased the intensity of this band still further. For p-nitrophenolate on silver, the polarity interaction and the resonance interaction should both act to increase the intensity of the ν8a band, while the ν8b band should be forbidden due to surface selection rules. (14) Merklin, G. T.; He, L.-T.; Griffiths, P. R. Submitted for publication in Anal. Chem. (15) Katritzky, A. R.; Topsom, R. D. Chem. Rev. 1977, 77, 639-58. (16) Wilson, E. B. Phys. Rev. 1934, 45, 706-14.
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The reduction in frequency of this band, which we observe in the surface-enhanced spectrum relative to that in the spectrum of the un-ionized phenol, may be attributable to an increase in single-bond character about the carbons to which the nitrogen and oxygen atoms are attached, as a consequence of the conjugated structure of the quinoid resonance form. We discussed above the assignment of the band at 1487 cm-1, discounting the possibility that it was the antisymmetric NO2 stretch. We believe that this band may result from the 1499-cm-1 phenyl ring mode, ν19a. This band was present, albeit only weakly enhanced, in the spectrum that we obtained in our study of nitrothiophenol. Unfortunately, neither the study of Epstein11 mentioned this band nor did that of Ni.12 Bellamy17 points out that this band, like its 1600-cm-1 counterpart, is substituent sensitive. However, unlike ν8a, where the substituents para to one another are moving in opposite directions, these are moving in the same direction. Thus one should not expect a strong donor/ acceptor pair to significantly increase the intensity of this band, even as it increases the intensity of ν8a. This conclusion is not consistent with our spectra if we assign the 1487-cm-1 band as ν19a, as the enhancement of these two bands in our experiments is roughly equal. The presence of several other strong bands nearby makes it difficult to draw conclusions about the mechanisms resulting in the presence, and enhancement, of this band. Comparison with Other Nitrophenols. 3-Nitrophenol. If the bands that are seen at 1270 cm-1 are a consequence of the stabilization of a quinoid resonance structure, then this band should be absent from a molecule where this resonance cannot take place. One such molecule is 3-nitrophenol, whose solution and surfaceenhanced spectra are shown in Figure 5. Note that there is little difference in the location and relative intensity of the two NO2 stretching bands, indicating that in this molecule, unlike the para isomer, there is no significant contribution from resonance forms that would reduce the frequency of the NO2 stretching modes. Also note that in this molecule the ν8 mode is almost entirely absent, unlike in the para form where the orientation of the electrondonating and -withdrawing groups act to increase the intensity of these bands. Dinitrophenols. A molecule such as 2,4-dinitrophenol might be expected to exhibit behavior similar to pnitrophenol, although perhaps to a lesser extent, given that more resonance structures are involved (see Figure 6). The solution spectrum of 2,4-dinitrophenol (Figure 7A) shows a band at 1267 cm-1 that is observed, albeit weakly, upon adsorption on silver. This is the band observed at 1260 cm-1 in the SEIRA spectrum (see Figure 7B). However, this band does not become as prominent in the SEIRA spectrum as the analogous band seen in the spectrum of p-nitrophenol. This may be because the ortho nitro group is oblique to the surface of the silver, reducing its interaction with the perpendicular electric field. The strong derivative shape of the neighboring band may also obscure its presence. It has been proposed by Ni et al.12 that 2,4-dinitrophenol adsorbs to the silver surface via coordination of the ionized NO2 group to the silver. It is argued that this is taking place because the hydroxyl group of the phenol will be sterically hindered by the nitro group ortho to it, to the extent that it will be unable to participate in chemisorption to the surface. Ni et al. claim that this is supported by the lack of interaction between the ortho isomer and silver (17) Bellamy, L. J. The Infra-red Spectra of Complex Molecules 3rd ed.; Chapman and Hall: London, 1975; Vol. 1.
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Figure 5. Spectra of 3-nitrophenol (A) on a 5-nm copper film, (B) on a 10-nm silver film, and (C) in CHCl3 solution.
Figure 6. Resonance forms for 2,4-dinitrophenol.
and argue that the quinoid resonance structure will give the nitro group para to the hydroxyl sufficient electron density to coordinate to the silver. We have also been unable to obtain SEIRA spectra of o-nitrophenol, which we attribute to its volatility in combination with internal hydrogen bonding that renders the hydroxyl group relatively unreactive. However, if the hydrogen bonding of the ortho isomer is also present in 2,4-dinitrophenol to the extent that it cannot chemisorb to the silver, similar behavior should be observed with the 2,5-dinitro isomer. In this molecule, the nitro groups are para to one another. If the molecule is adsorbing through a nitro group, then the molecule will be adsorbed with both nitro groups perpendicular to the surface of the metal film. If this is the case, then based on the orientation of the electric field to the surface of the film the dipole moment change of the antisymmetric NO2 stretching mode should interact only weakly with the electric field, which will be perpendicular to the metal surface. However, as is shown in Figure 8, the antisymmetric NO2 band at 1550 cm-1 is rather strong. This would imply that the molecule is adsorbed obliquely to the surface, through the hydroxyl group, with the nitro groups nearly parallel to the surface.
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Figure 7. Spectra of 2,4-dinitrophenol (A) on a 5-nm copper film and (B) in CHCl3 solution.
Furthermore, resonance effects will increase the electron density only of the nitro group ortho to the hydroxyl; theother nitro group is meta to the hydroxyl and cannot interact with it via resonance. If the 2,5-dinitro isomer could bind through the ortho nitro group, then one might expect the ortho mononitro compound to be able to do so as well, which conflicts with experimental observations. We have also obtained SEIRA spectra with 2,6-dinitrophenol and with 2,4,6-trinitrophenol; both spectra exhibited the characteristic absence of an OH stretch. We believe that despite the steric hindrance of the hydroxyl group, the dinitrophenols are still able to chemisorb through that moiety because of the higher reactivity of these molecules relative to the mononitro compounds. While the pKa of o-nitrophenol is 7.15, the pKa values of 2,5- and 2,4-dinitro isomers are 5.15 and 3.16, respectively, indicating their greater propensity (by 2 and 4 orders of magnitude, respectively) to release the acidic proton. We thus conclude that the nitrophenols (other than the ortho isomer) all chemisorb to the surface of the silver film through the hydroxyl oxygen.
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Figure 8. Spectra of 2,5-dinitrophenol (A) on a 5-nm copper film and (B) in CHCl3 solution.
Conclusions Differences in the relative enhancement for different bands in SEIRA spectra argue in favor of the possibility of chemical influences with respect to these bands. Nitro groups are strong electron-withdrawing groups, and it can be expected that they will participate in donoracceptor interactions when adsorbed onto a substrate. The consequences of these interactions are shifts in band location and intensity as a result of chemical changes in the adsorbed molecule. These changes are particularly pronounced when the nitro substituent is in a suitable position to engage in donor-acceptor interactions, as in 4-nitrophenol, and less so when the substituent is unable to do this, as in 3-nitrophenol. Acknowledgment. This work was supported through the National Science Foundation’s Experimental Program to Stimulate Competitive Research. LA960828S
